A grain-oriented electrical steel sheet is a soft magnetic material mainly used as a core material for transformers or the like, and is required to have excellent magnetic properties, particularly low iron loss. In the production of the grain-oriented electrical steel sheet, therefore, the steel sheet is subjected to a final annealing by heating to a high temperature of approximately 1000° C. to cause secondary recrystallization to thereby highly orient crystal grains in the steel sheet to Goss orientation ({110}<001> orientation). Moreover, in the final annealing, it is common to conduct a purifying treatment by heating to approximately 1200° C. to remove impurities, following the secondary recrystallization. Since the final annealing requires a long time of about 10 days at maximum, it is common to anneal the steel sheet with a coiled state in a batch type annealing furnace.
However, when the final annealing is conducted at such a high temperature over the long time, the coil itself is creep-deformed by its own weight or thermal expansion is restricted to cause a variety of shape defects, resulting in the decrease of the product yield. In the worst case, the steel sheet can no longer be threaded into a flattening annealing facility after the final annealing.
As a technique for solving such problems, various methods have been studied.
For example, Patent Document 1 proposes a technique wherein a thermal insulation material is lined to an inner side wall portion of an inner cover covering the coil in the final annealing to reduce wrinkle-like shape defects generated on the outer winding portion of the coil. Also, Patent Document 2 proposes a technique wherein an outer peripheral end face portion of a coil supporting stand in a final annealing furnace is covered with a thermal insulation material to prevent side distortion defects generated on the lower side portion of the coil contacting with the coil supporting stand. Further, Patent Document 3 proposes a technique wherein a metal ring is inserted into a central space of the coil placed at an upended state to prevent an inner winding portion of the coil from collapsing into the central space side.